How do I add a background image as a camera lens?
You can do this by adding a Background Image. To do this, go into the 3D Viewport and press the N key to open the panel on the right hand side. Scroll down to the very bottom, and you should see an option Background Images. Go ahead and enable it.



Now add an image and change the view from All Views to Camera.



Now in the camera view you will have a background image that transforms with the camera.